Field of the Invention
The present invention relates generally to chemical imaging and, more specifically, to microscopic and nanoscopic photo-thermal chemical imaging.
Description of the Prior Art
With the increasing materials complexity of microfabricated devices, there is a growing need for new characterization techniques that provide chemical composition with improved spatial resolution over relevant scales. Existing established techniques are not always well suited for the length scales involved in microfabricated devices. For example, FTIR spectroscopy provides averaged chemical composition information for millimeter sized samples but without spatial information. While FTIR micro-spectroscopy addresses this problem, the practical resolution limit is limited to about 20 μm. X-ray mapping can achieve higher resolution but provides elemental maps, though this is not very useful for identification of organic compounds. On the other hand, well-developed imaging techniques with nanometer resolution capability (e.g. SPM, AFM, TEM/EELS) are often impractical to operate at the micron-scale. These techniques, operated at the nanometer scale, typically offer a limited physical footprint scan range, operate well with samples exhibiting only a limited range of surface roughness, and routinely require sample preparation.
The emerging technique of Raman micro-spectroscopy provides adequate spatial resolution (˜1 μm). However, the process is inefficient and the signal response levels are extremely low, necessitating long integration times at each point, leading to long scan times and necessitating the use of expensive high performance detectors. In addition, samples which fluoresce cannot be practically characterized with Raman imaging.
The commercial techniques of combining atomic force microscopy and the photo-thermal effect provides chemical imaging with a spatial resolution as low as 0.1 μm. However, specialized sample preparation is needed, typically microtoming a sample to about 10 μm thin. Additionally, physical contact with both sides of the sample is required by a prism substrate and the scanning probe tip.
Photo-thermal spectroscopy (PTS) involves periodic heating of the sample and monitoring its response using either an IR detector or a visible probe beam (usually a HeNe laser). Photo-thermal IR imaging spectroscopy (PT-IRIS) for detection of chemicals at a distance has been implemented. In PT-IRIS, quantum cascade lasers (QCLs) are used to heat the sample and a long-wave IR detector is used as the imager. By varying the wavelength of the heating source across characteristic absorption bands the chemical composition of the sample is mapped out. However, the spatial resolution of this technique is diffraction limited by the long wavelength of the heating source and the thermal emission.
What is needed but not present in the prior art is a technique applicable to a wide range of samples, capable of mapping the chemical (molecular) information of the sample with spatial resolution better than 10 microns, and that is non-contact, non-destructive and requires limited or no sample preparation. The present invention satisfies all these criteria.